If you want to describe how an infectious disease spreads, one handy number is what epidemiologists call R0 (“R naught”), the disease’s basic reproductive number. As of now, estimates for the coronavirus’s R0 make it more contagious than the flu, but less than for many other infectious diseases.

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What is R0?

R0 is one of the numbers epidemiologists use to describe how an infectious agent spreads through a population. But it’s important to remember that it’s simply a statistic that describes some of the numbers we see.

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The basic idea is this: the average sick person, in a totally susceptible population, must be able to get at least one other person sick (R0 = 1) for the disease to spread. If a disease spreads to fewer than one person, on average, an outbreak can’t happen.

So, all epidemics involve something with an R0 of more than 1. Seasonal flu has an R0 of around 1.2. Measles has one of the highest R0 numbers, although it’s hard to pin down an exact number: 12 to 18 is typically cited. Many diseases, from the terrifying to the mundane, exist in the 2-7 range: Ebola, HIV, the common cold. The Guardian has a chart here that compares the R0 of several well known infectious agents, if you’re curious.

There are some important caveats about this number. First, it represents what happens in a population that is completely susceptible. Nobody has been vaccinated; nobody has had time to develop immunity. And second, it says nothing about how fast the disease will travel, just how many people it will eventually infect. Here’s a short video from a Penn State course on epidemics that describes R0 and how it relates to other information we need to know about how diseases spread.

So the researchers who have attempted to calculate the R0 for the current outbreak have had to make a lot of assumptions. As time has progressed, and as COVID-19 has spread further, we’ve started getting a better idea of what the R0 is. That said, this is still a rapidly changing situation, and numbers are likely to change as we learn more.

Maia Majumder, one of the scientists working on calculating the coronavirus’s R0, has an excellent twitter thread here explaining some of the assumptions her team used, and discussing how their estimates compare to other teams’. Keep in mind that her study was toward the beginning of the outbreak, when we didn’t know quite as much as we do now.

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That said, I know you’re here for the numbers, so here goes:

The current estimate of the World Health Organization is that the coronavirus has an R0 of 2.0 to 2.5. This is an estimate based on data reported as of March 6th, which, in pandemic time, was roughly five decades ago. Some studies of note include an analysis of the Diamond Princess Cruise ship, published in the International Journal of Infectious Disease, which estimates the R0 range to be within 2.06-2.52, with a median estimate of 2.28. Another study, this one published in the journal Science, that analyzed the early outbreak in China, estimates the initial R0 to be 2.38, before the travel restrictions were enacted.

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So far, these studies estimate the R0 to be somewhere in the 2.0-2.5 range. It’s important to remember that this is a situation-dependent number, one which depends on the environment, as well as the population of people it is infecting.

How much does R0 matter?

R0 is a useful number to understand when it comes to things like determining vaccine targets (the higher the R0, the more people you have to vaccinate to stop the disease from spreading). But the work of containing an outbreak can begin even before we have R0 nailed down.

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The R0 depends on a few things, including how long a person is contagious, how many susceptible people they tend to interact with, and how transmissible the infectious agent is.

That means we can make an epidemic less likely to spread by attacking those particular factors. We can make fewer people susceptible to the virus; that’s what a vaccine does. (There’s no vaccine for the coronavirus yet, but perhaps there will be someday.) You can reduce the chances of spreading the virus by practicing social distancing, quarantining yourself if you were exposed, and isolating yourself if you develop symptoms. You can reduce transmissibility through measures like hand washing and physical distancing.

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Editor’s note: This article, which was originally published in January 2020, was updated by Rachel Fairbank to include additional research, information, and recommendations for the coronavirus.